Group III nitride semiconductors, a typical example of which is gallium nitride (GaN), are wide-bandgap semiconductors about which, for example, gallium nitride (GaN) and aluminum nitride (AlN) have forbidden band widths of 3.4 eV and 6.2 eV, respectively, at room temperature. The group III nitride semiconductors are characterized by larger dielectric breakdown electric field and larger electron saturation velocity than gallium arsenide (GaAs) or such compound semiconductors, silicon (Si), and others.
Thus, as high frequency electronic devices or high power electronic devices, researches and developments of field effect transistors (FET) using a GaN-based compound semiconductor material have been actively made.
A nitride semiconductor material such as GaN is combined with AlN or indium nitride (InN) to give various mixed crystals. Thus, similarly to conventional arsenic-based semiconductor materials such as GaAs, the nitride semiconductor material can form a hetero junction. A hetero junction based on a nitride semiconductor, for example, an AlGaN/GaN hetero junction is characterized in that high-concentration and high-mobility carriers in its interface by spontaneous polarization and piezoelectric polarization are generated even in the state that the junction is not doped with any impurity. Thus, a transistor produced by use of the nitride semiconductor can be operated at a high speed.
Herein, AlGaN represents AlxGa1-xN wherein x satisfies the relationship 0<x<1, and InGaN represents InyGa1-yN wherein y satisfies the relationship 0<y<1. AlInN represents AlzIn1-zN wherein z satisfies the relationship 0<z<1, and InAlGaN represents InyAlxGa1-x-yN wherein x and y satisfy the relationship 0<x<1, 0<y<1, and 0<x+y<1. These notations are also applicable to the following description.
For example, unexamined Japanese Patent Publication No. 2008-251704 and unexamined Japanese Patent Publication No. 2011-103380 each disclose a technique of forming a plurality of nitride semiconductor layers over a silicon substrate.